Ionomer resins are resins having good toughness, sold under the trade name of Surlyn or Hi-miran, and used for cover of golf ball and the like. The resins contain free carboxylic acid moieties and, in the generally marketed resins, at least a part of such acid moieties are neutralized with metal ion. For example, in sodium neutralized type ionomer resins, 10-60% of the total carboxylic acid moieties are neutralized with sodium ion and the rest remains not neutralized.
Among ionomer resins, sodium neutralized type resins have excellent impact resilience but poor low-temperature durability, therefore, for example, there is the possibility of crack and the like at the temperature lower than -5 .degree. C.
To correct this defect, it is proposed that the sodium neutralized type ionomer resin is blended with a zinc neutralized type ionomer resin which has good durability at a low temperature. However, according to this blending method, fatigue failure upon repeated impacts may be accelerated. Moreover, the resins which can be mixed are limited to the zinc neutralized type ionomer resins, resulting in drawbacks, for example, limited freedom of design.
To avoid the problem caused by such blending methods, metal oxide is proposed to be compounded in sodium neutralized type ionomer resin to allow to react the residual free carboxylic acid moieties in the ionomer resin with desired metal (see, U.S. Patent Ser. Nos. 4526375 and 662322 and PCT Application No. 8702039).
However, the above process is disadvantageous because reactivity is insufficient to carry out mixing and reacting in an extruder.